Asp.net core + IIS Express. How to view log messages?

You can use the Debug listener to achieve what you want:

"dependencies": {
  "Microsoft.Extensions.Logging.Debug": "1.0.0-rc1-final"
}

---

Console.WriteLine( "Hello World!" );

loggerFactory.MinimumLevel = LogLevel.Debug;
loggerFactory.AddDebug( LogLevel.Debug );
var logger = loggerFactory.CreateLogger("Startup");
logger.LogWarning( "Hi!" );

IIS Express will not output its logs to Visual Studio's output window.

You'll need to use a different log provider.

Write it to a log file. You can use a library for that, for example Serilog:

using Serilog;

public class Startup
{
  public Startup(IHostingEnvironment env)
  {
      Log.Logger = new LoggerConfiguration()
          .WriteTo.File("log.txt")
          .CreateLogger();
...
public void Configure(IApplicationBuilder app, IHostingEnvironment env,
                        ILoggerFactory loggerFactory)
  {
      loggerFactory.AddSerilog();

You'll need this NuGet package.

Go to Documents => IISExpress => Logs.

This is what you are looking for.